Add option to keep current source on lang change

Closes #1604

static/panes/editor.js

@@ -944,7 +944,7 @@ Editor.prototype.onLanguageChange = function (newLangId) {
         if (newLangId !== this.currentLanguage.id) {
             var oldLangId = this.currentLanguage.id;
             this.currentLanguage = languages[newLangId];
-            if (!this.waitingForLanguage) {
+            if (!this.waitingForLanguage && !this.settings.keepSourcesOnLangChange) {
                 this.editorSourceByLang[oldLangId] = this.getSource();
                 this.updateEditorCode();
             }

static/settings.js

@@ -229,6 +229,7 @@ function setupSettings(root, settings, onChange, subLangId) {
     add(root.find('.editorsFLigatures'), 'editorsFLigatures', false, Checkbox);
     add(root.find('.allowStoreCodeDebug'), 'allowStoreCodeDebug', true, Checkbox);
     add(root.find('.useVim'), 'useVim', false, Checkbox);
+    add(root.find('.keepSourcesOnLangChange'), 'keepSourcesOnLangChange', false, Checkbox);
 
     setSettings(settings);
     handleThemes();

views/popups.pug

@@ -145,6 +145,10 @@
                 label
                   input.showMinimap(type="checkbox")
                   | Show editor minimap
+              .checkbox
+                label
+                  input.keepSourcesOnLangChange(type="checkbox")
+                  | Keep editor source on language change
               div
                 label
                   |  Tab width